Focusing and deflecting system for a cathode ray tube



June 10, 1969 MAY ETAL- 3,449,624

FOCUSING AND DEFLECTING SYSTEM FOR A CATHODE RAY,TUBE

Filed Sept. 15, 1966 Sheet of 2 United States Patent US. Cl. 315-31 5 Claims ABSTRACT OF THE DISCLOSURE A cathode ray tube having electrostatic focusing and deflecting systems has in order between the gun cathode and the target, first and second pairs of opposed electrodes symmetrically arranged relative to the axis of the tube, a mesh electrode perpendicular to the axis of the tube and a further pair of opposed electrodes symmetrically arranged with respect to the axis of the tube. The second pair of electrodes is supplied with potentials for deflecting the beam in a first plane including the axis of the tube, and the further pair of electrodes is supplied with potentials for deflecting the beam in a second plane including the first axis and perpendicular to said first plane. Moreover, the first and second pair of opposed electrodes are also suitably energised to form a first cylinder electrostatic lens for focusing the electrons in said first plane to a line perpendicular to said plane. The mesh and the further pair of opposed electrodes cooperate to form a second cylinder electrostatic lens for focusing the electrons in the second plane, to produce in the absence of the first cylinder electrostatic lens a line perpendicular to said second plane. The mesh is positioned adjacent the exit end of the second pair of opposed electrodes to control the curvature of the image field.

This invention relates to cathode ray tubes and to electron beam focusing and deflecting arrangement incorporating such tubes. The invention is especially ap plicable to television pick-up tubes of the vidicon or similar type having a photoconductive target.

One object of the present invention is to produce an improved cathode ray tube including electrodes of a combined electrostatic focusing and deflecting arrangement for the electron beam of said tube.

Another object of the invention is to provide an improved electrostatic focusing and deflecting arrangement for the electron beam of a cathrode ray tube incorporated in the arrangement.

A further object of the present invention is to provide an improved electrostatic deflecting arrangement for the electron beam of a cathode ray tube and including electrode means for electrostatically improving the focus of the beam.

According to the present invention, from one aspect, there is provided a cathrode ray tube having between the cathode and the target thereof a first electrostatic deflection system adapted to deflect electrons from the cathode in a first plane including the tube axis, electrode means between said cathode and said first system capable, when suitable potentials are applied, of cooperating with said first system to form a first cylinder electrostatic lens for focusing said electrons in said first plane to a line perpendicular to said first plane, a second electrostatic deflection system nearer said target than said first system and adapted to deflect electrons in said second plane including the tube axis and perpendicular to said 3,449,624 Patented June 10, 1969 first plane, and a substantially continuous but electronpervious electrode substantially perpendicular to the undeflected path of the electrons and so arranged between said systems as to be capable, when suitable potentials are applied, of cooperating with said second system to form a second cylinder electrostatic lens for focusing said electrons in said second plane, to produce in the absence of said first cylinder electrostatic lens a line perpendicular to said second plane.

The present invention also includes the incorporation of a cathode ray tube, such as described in the preceding paragraph in a circuit for applying appropriate deflecting and energising potentials to the electrodes of the tube.

According to the present invention from another aspect there is provided an electron beam focusing and deflecting arrangement for the electron beam of a cathode ray tube, comprising a cathode ray tube having between the cathode and target thereof an electrostatic deflection system including a pair of deflecting plates connected to a source of deflecting potential for deflecting electrons from said cathode in one plane including the tube axis, a substantially continuous but electron-pervious electrode substantially perpendicular to the undeflected path of said electrons and positioned adjacent the exit edges of said plates, said electron-pervious electrode being connected to a source of potential and arranged when energised by said source to control curvature of the image field so as to substantially improve the focus of the electrons over the area of the target.

The expression substantially continuous electronpervious electrode" is intended to mean an electrode such as a mesh of small pitch, which though allowing the passage of the beam, provides a substantially continuous equipotential surface.

In a preferred example of the invention, on the side of the electron-pervious electrode nearer the source of the electron beam, two pairs of plates are arranged in succession in the path of the beam so that by the application of suitable potentials to the two pairs of plates they interact to form the first cylinder electron-lens, the deflection being produced by the pair of plates nearer the mesh so that most of the deflection occurs after the focusing. In this form of the invention, preferably, .the nearer pair of plates may conveniently be arranged to coact with the electron-pervious electrode to form a further electrostatic field for controlling or compensating for the curvature of the image surface.

In order that the present invention may be clearly understood and readily carried into effect, it will now be described with reference to the accompanying drawings, in which:

FIGURE 1 illustrates diagrammatically and in simplified form one example of a cathode ray tube according to the invention,

FIGURE 2 is a diagram, based on FIGURE 1, illustrating the functioning of the tube shown in FIGURE 1 when incorporated in an appropriate external circuit to form an operational focusing and deflecting arrangement. In this diagram, for clarity, the plates 6 have been shown rotated by about the tube axis.

FIGURE 3 is a further diagram illustrating the functioning of the tube of FIGURE 1,

FIGURE 4 illustrates a modification which may be made in the FIGURE 1 arrangement, and

FIGURES 5a and 5b illustrate respectively two parts of a preferred form of an electrostatic focusing and deflecting arrangement according to the invention and incorporating a cathode ray tube generally similar to that illustrated in FIGURE 1.

Referring to FIGURE 1, it will be assumed that the arrangement illustrated therein constitutes a television pick-up tube having a photoconductive target; represented by the reference 1 and an electron gun represented diagrammatically by the reference 2. The electrodes are enclosed in an envelope, which includes also a wall anode and ion trap mesh-electrode in known manner. The envelope, wall anode and ion-trap mesh are not shown as they are conventional. The electrostatic focusing and deflecting electrodes comprise a first pair of plates 3, a second pair of plates 4, a mesh and a third pair of plates 6. The plates 3 are parallel to the plates 4 and in operation of the arrangements, the plates 4 are the vertical deflecting plates of the cathode ray beam. The plates =6 though they may be slightly divergent as indicated, are generally perpendicular to the plates 4 and in operation of the arrangement produce the horizontal deflection of the beam. The mesh 5 is a fine pitch mesh and its plane is perpendicular to the axis of the tube.

An external circuit is provided for applying appropriate potentials to the plates and such circuit maintains a relatively low potential at the plates 3, a relatively high mean potential on the plates 4, a potential on mesh 5 equal, in this example, to said mean potential and a relatively low mean potential on the plates 6. The potentials on the plates 4 and 6 are in addition variable in push-pull manner to produce the required mutually perpendicular deflections, the plates being connected to conventional circuits to achieve this function. By virtue of the potential difference between the plates 3 and 4, the first cylinder lens is formed by curvature of the electropotential surfaces in the region 7, as indicated in FIGURE 2. The overall effect of this lens is convergent. Moreover by virtue of the potential difference between the mesh 5 and the plates 6 a second cylinder lens is produced in the region 8 which is also a convergent lens. In FIGURE 2, the lens 8 is shown rotated through 90 from its correct position around the tube axis so as to simplify the illustration. The two cylinder lenses 7 and 8 are in fact crossed and serve to bring the beam substantially to a point focused on the plane of the target 1. The plates 4 are placed relatively close together to save space, and to achieve good deflection sensitivity even though the mean voltage on the plates is relatively high. However close plates are likely to need correction for curvature of field, but such correction is in the present case produced by the interaction of the mesh 5 and the plates 4, said correcting being computed to control or compensate at least partly for curvature of the image surface.

The correction of image curvature by the mesh 5 and the plates 4 can be explained by reference to FIGURE 3. In one mode of operation the mesh 5 is maintained at the mean potential of the plates 4. When a small or no deflecting voltage is applied then little or no lens action exists between the plates 4 and the mesh 5. This corresponds to the points P and P in FIGURE 3. However when a deflecting voltage is applied the equipotential system shown diagrammatically by the lines S is set up. The electrons passing through the points Q and Q are affected by the lens formed by the curved equipotentials and the focused line is moved further from the mesh 5. The effect is non linear with distance of pencil from mid plane. A similar action can take place even though the plates 4 may not operate at the same potential as mesh 5, for in this case the varying lens already described is superimposed on the fixed lens now formed between plates 4 and mesh 5. The correcting effect can wholly or partly compensate or over-compensate for the curvature which is introduced by the deflecting plates following the lens 7. Moreover the amount of correction can be varied by varying the gap g between the exit edge of the plates 4 and the mesh 5, or by other geometric variations or by varying the potentials. The gap g may be of the order A to of the separation of the deflecting plates 4 The lens in the region 7 is effective at the predeflection end of the plates 4, and errors are avoided such as would be produced if the main focusing effect occurred between 4 and 5. Moreover, the second crossed cylinder lens in the region 8 is at the predeflection end of the plates 6 and so gross errors are also avoided. However, such correction of curvature as can be produced by interaction of the mesh 5 and the plates 6 is very much smaller than that which can be produced by the plates 4 and the mesh 5 because the mesh is at the pre-deflection end of the plates 6 so that the two pencils corresponding to P P and Q and Q; are, near to the mesh, very close together near the axis. It is desirable that the plates 6 are sufficiently far apart to avoid undesirable edge effect and instead of being slightly divergent as indicated, they may be parallel, or stepped, as shown in FIGURE 4. If desired a second mesh may be placed after the plates 6 for correction purposes.

FIGURES 5a and 5b illustrate respectively the horizontal and vertical deflecting and focusing parts of an arrangement for the electron beam of a tube which is similar in principle to that illustrated in FIGURE 1. Corresponding parts in FIGURE 1 and FIGURES 5a and 5b are denoted by the same references. In FIGURES 5a and 5b, the external circuit connections as such are not shown as they are conventional, but are represented by the potentials and potential variations which they provide. The range of variation of the potentials applied to the plates 4 and 6 by the deflecting circuits, as indicated, are the double amplitude of the push-pull deflecting waveforms which are applied respectively to these pairs of plates. All potentials indicated on the drawing are in volts. As already indicated any suitable system of electrodes can be arranged to follow the plates 6 within the tube envelope, including electrodes required to produce normal incidence of the beam on the target.

Although the systems described employ planar deflecting electrodes, these may be curved in either or both planes. Curvature in the plane of deflection may be used to reduce aberrations or increase deflection sensitivity. Curvature in the perpendicular direction may be used to reduce edge effects or image surface distortion. Similarly the mesh electrode 5 and the corresponding electrode after the plates 6 may be curved, and it is to be understood that the expression substantially perpendicular to the undeflected path of the electrons used herein and in the claims is intended to include electrodes such as 5 which are thus curved.

We claim:

1. A cathode ray tube having between the cathode and the target thereof a first electrostatic deflection system adapted to deflect electrons from the cathode in a first plane including the tube axis, electrode means between said cathode and said first system capable, when suitable potentials are applied, of cooperating with said first system to form a first cylinder electrostatic lens for focusing said electrons in said first plane to a line perpendicular to said first plane, a second electrostatic deflection system nearer said target than said first system and adapted to deflect said electrons in a second plane including the tube axis and perpendicular to said first plane, and a substantially continuous but electron-pervious electrode substantially perpendicular to the undeflected path of the electrons and so arranged between said systems as to be capable, when suitable potentials are applied, of cooperating with said second system to form .a second cylinder electrostatic lens for focusing said electrons in said second plane, to produce in the absence of said first cylinder electrostatic lens a line perpendicular to said second plane.

2. A tube according to claim 1, in which said first electrostatic deflection system comprises a pair of opposed electrodes symmetrically arranged relative to the axis of the tube, and said electrode means between said cathode of said first system comprises a further pair of opposed electrodes, symmetrically arranged relative to the axis of the tube, the adjacent edges of the pairs of the plates being substantially parallel.

3. An electron beam focusing and deflecting arrangement for the electron beam of a cathode ray tube, comprising a cathode ray tube having between the cathode and the target thereof a first electrostatic deflection system coupled to a deflecting circuit for deflecting electrons from said cathode in one plane including the tube axis, electrode means between said cathode and said first system connected to a source of potential and arranged When energised by stid source to cooperate with said first deflection system to form a cylinder electrostatic lens for focusing said electrons in said first plane to a line perpendicular to said first plane, a second electrostatic deflection system nearer said target than said first system and coupled to a deflecting circuit for deflecting said electrons in a second plane including the tube axis and perpendicular to said first plane, a substantially continuous but electron-pervious electrode substantially perpendicular to the undeflected path of the electrons, said electron-pervious electrode being connected to a source of potential, and so arranged between said system that when energised by said latter source it cooperates with said second system to form a second cylinder electrostatic lens for focusing electrons in said second plane, to produce in the absence of said first cylinder electrostatic lens a line perpendicular to said second plane.

4. An arrangement according to claim 3 in which the electron-pervious electrode is positioned adjacent the exit end of the first electrostatic deflection system to control the curvature of the image field.

5. An electron beam focusing and deflecting arrangement incorporating a cathode ray tube having between the cathode and target thereof an electrostatic deflection system including a pair of deflecting plates connected to a source of deflecting potential for deflecting electrons from said cathode in one plane including the tube axis, a substantially continuous but electron-pervious electrode substantially perpendicular to the undeflected path of the electrons and positioned adjacent the exit edges of said plates, said electron-pervious electrode being connected to a source of potential and arranged when energised by said source to control curvature of the image field so as to substantially improve the focus of the electrons over the area of the target.

References Cited RICHARD A. FARLEY, Primary Examiner.

M. F. HUBLER, Assistant Examiner.

US. Cl. X.R. 313-78, 

